1. Field of the Invention
This invention relates to the continuous casting of metals and more particularly to an improved method of and apparatus for controlling the level of molten metal in the mold of a continuous caster.
2. Description of the Prior Art
In the continuous casting of metal, it is important to maintain the level of the liquid metal in the mold at the optimum level both to obtain maximum production within the mold's capacity and to insure against inferior product or excessive down time which can result from excessive deviation from the optimum metal level and which can produce breakouts or other failures. Numerous devices have been developed for continuously monitoring the level of liquid metal in the caster mold, and for automatically adjusting the metal level upon an excessive deviation being detected. The metal level has been controlled by varying the withdrawal rate of the cast strand, by adjusting the flow rate of liquid metal from the tundish to the caster mold, or by a combination of these two methods. Initially maintaining the molten metal at the desired level was under control of an operator who monitored the mold and made the necessary adjustments, one such system being disclosed, for example, in U.S. Pat. No. 2,832,110.
More recent developments for monitoring and controlling the molten metal level in a continuous caster mold have included computerized control systems which are more reliable and more responsive to small deviations from the desired level. Such systems have employed automatic metal level sensing devices including thermocouples in the mold wall, eddy current sensors and optical devices which function to detect and signal deviations from a reference liquid level. The level deviation signal is then employed to regulate the withdrawal speed of the strand or to adjust a throttling valve opening to control the flow of metal from the tundish into the mold. For example, U.S. Pat. No. 3,946,795 discloses a system employing a radio-active emitter and receiver for detecting deviations from a predetermined level both in the positive and negative directions and signalling the measured deviations to a controller which adjusts the withdrawal speed of the strand as required to bring the level of liquid metal back to the desired level.
U.S. Pat. No. 3,300,820 discloses a system which includes a molten level detecting apparatus which develops a DC signal whose polarity is an indication of the direction of deviation from a preselected level and the magnitude of the signal is an indication of the extent of such deviation. The error signal is employed to adjust the position of the stopper rod in the tundish outlet nozzle by energizing a solenoid in a hydraulic control system to adjust the stopper in the appropriate direction. A time delay relay is connected in the circuit controlling the hydraulic system to minimize overshooting or hunting of the system.
In addition to sensing the molten metal level, it is also known to continuously sense the position of the tundish sliding gate valve or stopper valve (throttling valves) and to utilize a computer to calculate the open area through which the molten metal flows into the mold. The open valve area and measured metal level deviation can then be utilized to calculate a new open area desired to correct any deviation from the desired mold level, and to actuate the throttling valve for a predetermined time which the computer determines to be necessary to produce the new area.
While the prior art systems of the type described above have been utilized with more or less success, they have not been entirely satisfactory for various reasons. For example, when a sliding gate is employed to control flow from the tundish, a circular opening in the gate overlies a corresponding circular opening in the bottom wall of the tundish, with the two being completely aligned for maximum flow. As the sliding gate is moved to restrict flow, the open area varies with sliding movement in accordance with a complex formula, producing a continuously changing geometry for the net opening which, in turn, produces flow restrictions which vary with the fluid flow characteristics and the liquid metal level in the tundish. Further, gate or stopper movement is not always precise as a result of various factors including sliding friction, variations in hydraulic pressure in the actuating system, gases in the hydraulic fluid and the like. Such systems also have generally required complex and expensive actuating equipment in order to produce as closely as possible the desired movement. Nevertheless, such known systems are subject to substantial hunting, overshooting and time lag, with the result that the mold level is not maintained with the desired degree of consistency. Accordingly, it is a primary object of the present invention to provide an improved method of and apparatus for controlling the level of liquid metal in a continuous casting mold.
Another object of the invention is to provide such a method and apparatus which maintains the desired liquid metal level with substantially greater accuracy by reducing overshoot or hunting by the system.
Another object is to provide such a method and apparatus employing a computer controlled actuating system for operating a slide gate throttling system for a tundish or ladle including both high and low speed actuating hydraulic systems for positioning the throttling gate to control the flow of metal and maintain the proper level.
Another object is to provide such a method and apparatus in which the rate of movement of the throttling gate is sensed and fed to the computer which utilizes the rate signal to more accurately determine the gate movement cutoff, thereby providing more accurate control of metal level in the mold.